1. Field of the Invention
The present invention relates to a sound absorption and reflection material, made from a particular elastomeric, syntactic polyurethane mixed with polymeric microspheres which is useful as a castable, moldable or sprayable composition on a wide variety of articles and substrates.
2. Description of the Prior Art
There are many instances where materials having good acoustic absorption and acoustic reflection characteristics would be valuable for sound reduction and sound scattering. Such material would be useful for commercial sound deadening applications in offices and the like. Such materials would also be useful for noise reduction applications for a variety of ocean going vessels. There is a prime need to suppress or control unwanted sound in underseas and surface vessels so as to reduce their detectability.
One method for reducing unwanted sound is to coat the object or substrate with some combination of materials effective to achieve quieting from background noise within a container and in some instances from incoming signals, such as sonar. Some coatings useful in this area are: decoupling coatings which will contain sound within the structure encased by the coating and limit sound radiation to the outside, and anechoic coatings which reduce the echo of an incident sonar signal by, for example, absorption or scattering.
Effective sound absorption and reflection coatings present the greatest difficulties for submergible vessels. We have found that the primary required materials properties include: low density coupled with compressive stiffness high enough to limit compressibility at full depth to small levels. The materials must be able to withstand broad temperature and pressure ranges in water over many cycles of application without displaying a significant level of permanent distortions or compressive set, or loss of adhesion to their substrate. The materials must have high damping with low frequency sensitivity so as to absorb sound vibrations over broad frequency ranges. The materials must have relative insensitivity to temperature changes and moisture, in order to be durable and have good acoustic properties over the whole span of applicable frequencies.
Ordinarily, materials that exhibit temperature insensitivity over large temperature ranges can be shown to be correspondingly insensitive to frequency, using time temperature superposition principles. Appropriate polymeric or other type materials must be capable of application as either cured cast tiles, mold-in-place compositions, or sprayable compositions.
Polyurethane elastomers are well known for demanding and sophisticated uses in marine and aerospace applications. Marine applications are taught in U.S. Pat. No. 3,632,703 (Sullivan et al.), relating to controlled gas entrapment in epoxy, or to a lesser extent, polyurethane resins, by means of encapsulation of either unbreakable plastic, or glass microspheres enclosing a vacuum or a gas, for use as an acoustic window in torpedo transducers. Here, the capsule-resin matrix is precompressed to deform or shatter the capsules, to provide a structural material having a product of velocity of acoustic propagation and density equal to that of sea water.
Aerospace applications are taught in U.S. Pat. Nos. 4,604,940 and 4,485,719 (Mendelsohn et al.), relating to flexible shock isolator pads and relatively rigid elastomeric launch seals, respectively. These materials were prepared from diol plus triol based diphenylmethane diisocyanate prepolymers extended with hydroquinone di(.beta.-hydroxyethyl) ether. Such materials, however, do not meet the basic requirements previously outlined and the use of the hydroquinone ether diol requires relatively high processing temperatures, which are impractical for casting in place against substrate or spray applications.
Use of microsphere filled, light weight polyurethanes for aircraft, missile, and other applications is taught in U.S. Pat. No. 3,472,798 (Pitchforth et al.). Here, from 5 wt. % to 50 wt. % of polyvinyl chloride is utilized to stabilize a suspension of polymethylmethacrylate microspheres containing neopentane gas, in a polyglycol mixture suitable for reaction with a dipropyleneglycol-toluene diisocyanate adduct and a catalyst. Density of the cured material ranges from 0.78 g/cm.sup.3 to 1.15 g/cm.sup.3, with a hardness, shore A, of 65 and 75, and tensile strengths of 496 psi and 805 psi respectively. The lower density materials are relatively soft. The use of polyvinylchloride, inert to the polyurethane, would detract from physical properties and possibly decompose to HCl gas on exposure to moderate heat. Lower density polyurethanes are taught in U.S. Pat. No. 4,038,238 (Cravens), where densities in the range of 0.5 g/cm.sup.3 to 0.6 g/cm.sup.3 are achieved. Here, xylene, toluene and the like are used to reduce viscosity in combination with glass or polymeric, hollow microspheres, dioctyl phthalate and a rapid polyurethane-forming composition The dioctyl phthalate acts as a plasticizer, degrading physical properties as does the xylene type materials which could easily diffuse from the cured structure so that physical properties could change over time.
What is needed is an acoustic absorption and low reflection material that displays very low sensitivity to wide changes in frequency and temperature and which resists the effects of high external pressure, as well as exhibiting good hydrolytic stability and excellent elevated temperature stability. The need for such a material has been generally known for some time. It is one of the main objects of this invention to provide a composition curable to such a material.